SPECIATION OF EDTA IN NATURAL-WATERS - EXCHANGE KINETICS OF FE-EDTA IN RIVER WATER

In order to understand the speciation of EDTA, the exchange kinetics of Fe(lll)-EDTA with other cations (Zn, Ca) are studied under the conditions of natural river water. Measurements in sewage treatment plant effluents indicate that EDTA is partly present as Fe(lll)-EDTA. At equilibrium, Fe-EDTA introduced into river water is expected to exchange with Zn(ll) and Ca to form Zn-EDTA or Ca-EDTA. The exchange of Fe-EDTA with Zn(ll), which was,followed by voltammetric measurement of labile Zn, as well as the disappearance of Fe-EDTA, which was measured asa photolabile species, occur on a time scale of days. The: results are presented in terms of second-order rate laws, with the average observed rate constant, k = 10.3 M(-1) s(-1) for the exchange of Zn(ll) with Fe-EDTA in river water; the dissociation of Fe-EDTA appears to be the rate-limiting step for-this reaction, with a first-order rate constant of 4 x 10(-7) s(-1). The exchange of Zn(ll) with Ca-EDTA is much faster, with k = 1.1 x 10(3) M(-1) s(-1). From the first-order rates, a half-time of about 20 days is obtained for the exchange of Fe(lll)-EDTA in the river water, If a slowly dissociating complex, such as Fe-EDTA, is initially present, the initial speciation is determining the fate of EDTA in river water; equilibrium speciation:of EDTA and trace metals may not be reached within the time scale of the river flow if the inputs occur as Fe-EDTA.